INDIAN WELLS, CALIF.—“He’s not what he used to be, is he? It’s hard to watch.”—Roger Federer fan in section 172 in Stadium 1 at approximately 4:00 P.M. on Wednesday.

“He’ll never be the same player, not after everything with his knees.”—Rafael Nadal fan in section 172 in Stadium at approximately 8:00 P.M. Wednesday.

“I mean, we’re both a bit suspect going into our match, I guess, so it’s an interesting matchup. In the past, this match used to be a final, now it’s a quarterfinal.”—Roger Federer, after his win over Stan Wawrinka, on the possibility of playing Nadal on Thursday.

Poignant quotes, aren’t they? Time flies in tennis, if you haven’t noticed, and the players who once seemed invincible become “suspect,” and “not what they used to be,” in the space of half a decade—which can feel more like the blink of an eye. Federer and Nadal once had the No. 1 and No. 2 rankings on lockdown; now Nadal is No. 5 and Federer could soon go from No. 2 to No. 3. It once seemed a tennis fan's God-given right that all of their matches would be epic finals; tomorrow, as Federer said, they’ll be facing each other in the quarterfinals for the first time in their 29 meetings. Coming in to it, the theme for Federer-Nadal XXIX might be “creeping vulnerability.”

For better and worse (and then better again), Federer’s 6-3, 6-7 (4), 7-5 win over Wawrinka today had all of the hallmarks of his play over the last few years. There was, first of all, the bad back, which has plagued him at various times in his career, and which forced him to cancel his practice session the previous day. Federer said afterward that it felt better than it had on Tuesday, and that he expected it to continue to improve on Thursday.

Still, there was, despite that niggle, his usual strong start. Federer broke twice in the first set, and he handled Wawrinka with tactical mastery for the better part of two. Much has been made, rightfully, of Stan's second-fiddle, little brother status to the Swiss Maestro, but Federer also happens to be an old-fashioned bad matchup for him. Wawrinka is a rhythm player, and Federer, obviously, can mix up his shots and keep an opponent from getting any rhythm. He did that well today. Federer followed the flat serve into the body with a slow kick wide; a drive forehand was followed by a high one with heavy topspin, which led to a short slice backhand. By the middle of the second set, it looked like Stan, who was playing at a more casually fast clip than normal, was ready to accept his 15th defeat in 16 matches against his doubles partner.

But while fast starts and tactical smarts have always been part of the Federer repertoire, as he’s aged they’ve been joined by a more troubling trait: an inability to close. In the last few years, Federer has lost numerous times after holding match points, including last month to Tomas Berdych in Dubai. This time he didn’t go quite that far, but he was, out of nowhere, broken at love while serving for the match at 5-4 in the second. Still, Federer found a way to survive. Chalk at least a little bit of it up to that other helpful aspect of his game: Who he is. As soon as Wawrinka got a lead at the start of the third set, he gave it right back.

As for Nadal, we know what injury he came here with, and it was clear that he wasn’t moving comfortably in his win over Ernests Gulbis. He said afterward that his knees were “so so,” and that he was unable to play this opponent exactly the way he wanted.

“Against a player like Ernests,” Rafa said, “the right thing to do, in my opinion, is play against his backhand and change to his forehand quick, no? But today I didn’t have the legs for that...I felt if I changed directions I had to run more, too. I didn’t want that today.”

In the end, Rafa won the same way Roger had, with a stirring late run that led to a 4-6, 6-4, 7-5 victory. And he did it in characteristic fashion as well. For much of the third, Nadal had been content to loop his forehand to Gulbis’ backhand; when he was nervous, those shots began landing shorter and shorter. But just as Rafa appeared to have reached the end of his rope while serving at 4-5, two points from defeat, he flicked a backhand pass and came up with first serves to hold. In the next game, he left the nerves behind—or more accurately, he stepped past the nerves—and hit his best, flattest, fastest forehand of the night to reach break point. Nadal kept stepping forward to serve out the match.

Rafa was asked afterward about his memories of his first encounter with Federer, in Key Biscayne in 2004, when he was 17.

“That moment was my best match ever,” Nadal said with a smile today. "Feels like it happened 100 years before.”

Rafa and Roger both sound a little older and a little wearier at the moment. They likely will never be No. 1 and 2 again, and they may never stage another epic in a Grand Slam final. But if they revealed their weaknesses today, they also made us remember why, in comparison to past tennis champions, they have endured. They took their creaky bodies and creeping vulnerabilities and won anyway. In the end, we got to see them the way we want to see them—Rafa with his fist raised in cathartic victory, ratcheting up the crowd’s noise level; Roger pushing his hair back for his post-match interview, a wide smile on his face as 16,000 people cheered.

Maybe it’s not such a bad thing that they can meet in quarterfinals now; if they stay ranked where they are, we might see them face off more often. This is what Federer said, right after he claimed that both of them were “suspect" this week:

“It’s alway very exciting playing each other,” he said. “Obviously playing Rafa, it’s a classic.”